The invention is in the field of therapeutic ultrasound apparatus and methods.
Ultrasound has been used in various diagnostic procedures, including the visualization of the interior structure and valvular operation of the human heart. Ultrasound has also been used in certain therapeutic procedures, such as for muscle heating. Examples of this type of therapeutic use of ultrasound may be found in U.S. Pat. No. 3,735,756 to Richards et al and in U.S. Pat. No. 3,828,769 to Mettler. Heretofore, to applicant's knowledge, a physician-controlled treatment system has not been proposed to utilize ultrasound apparatus in the treatment of post-myocardial infarct.
Despite advances in the understanding, diagnosis, and treatment of myocardial infarction, this disease process still ranks as the number one cause of death in much of the world today. It has been only in the last decade or so that investigators have met with success in trying to understand the problems and have been able to devise treatment regimens based on adequate scientific investigation. This is evidenced by the success of coronary care units in monitoring and treating arrythmias resulting from electrical malfunctions, so that death from such malfunctions now can be nearly abolished.
More recently, efforts have been directed toward limiting the size of infarcted myocardium and enhancing the healing process. Such efforts are based on the experimental finding that in ischemically injured myocardium there is generally a central zone of irreversibly injured myocardial cells and a surrounding zone of reversibly injured cells that survive the acute episode. In ny of the occulsive processes that lead to ischemic damage, the possible role of cell swelling resulting from the ischemia recently has been recognized as a mechanism of secondary damage. When coronary blood flow is restricted, the metabolism of the myocardial cells is inhibited so that the active processes which require energy are also inhibited as soon as the energy stores are depleted. The ionic concentrations which are maintained by active transport mechanisms are upset. The intracellular osmotic pressure increases as the sodium concentration increases, and thus the cells swell. It has been suggested that the resulting edema actually may contribute to the restriction of blood flow, thus compounding the trauma to the myocardium.
Low intensity ultrasound has been used in the past in physical medicine for combatting edema and the swelling of tissues. It has been demonstrated that ultrasonic irradiation increases the permeability of synovial membranes; and it has also been found that ultrasound increases the permeability of membranes in a dialysis unit. Recent studies suggest the possible beneficial effects of low intensity ultrasound on the processes of regeneration and repair of injured tissues. For example, it has been reported that there was an increased rate of healing in wounds treated for five minutes three times per week with low intensity pulsed ultrasound. One study has shown that there is less necrosis, less fibrosis, and increased vascularization in acutely infarcted myocardium irradiated with ultrasound (810 KHz, 1 w/cm.sup.2) for eight treatments during an 18-day period.
In the infarcted heart, circulation is at its lowest ebb at the very time when the demands for transport are the greatest, i.e. when degeneration of heart muscle and subsequent regeneration of scar tissue are taking place. During the height of the degenerative processes, the environment within the lesion is not conducive to the survival of normal tissue components.
Apparently, small vessels are lost during this stage inasmuch as scar tissue normally exhibits far less vascularity than normal myocardium, thus any treatment mode which favors the survival or development of circulation within the lesion contributes a most desirable factor for the repair process. Ultrasound has been shown to be capable of producing flow of interstitial fluids resulting from radiation pressure. Thus ultrasonic treatments can provide a form of circulation during the most critical stages of degeneration so that small blood vessels within the muscle can survive as well as promote the proliferation of new vascularity into the infarcted region. Swelling of cells in the vicinity of the lesion as previously noted tends to further compromise the already traumatized tissue, and it has been shown that low level ultrasound is effective in increasing the permeability of the cellular membranes with a concomitant reduction of cell volume in edematous tissue. Insofar as these benefits accrue to cells of the myocardium compromised by myocardial infarction, the trauma will be materially reversed for the patient.